1. Field of the Invention
The present invention relates to a nonvolatile switch, in particular for high-density nonvolatile programmable-logic devices.
2. Description of the Related Art
As known, programmable-logic devices are currently mainly formed by RAMs, which must be written each time the device is turned on. It is therefore necessary to provide an external memory that contains the code to be loaded at turning-on.
To eliminate the above need, programmable-logic devices, based upon nonvolatile components, have already been proposed. A solution is disclosed in U.S. Pat. No. 5,015,885, wherein a nonvolatile cell (EPROM or EEPROM) operates directly as a switch for connecting or separating horizontal and vertical segments formed by pass transistors. However, this solution is problematical as regards management of the switches, since they carry out two different functions and hence require a separate encoding for each function.
In other solutions, disclosed for example in U.S. Pat. NO. 6,625,221 and U.S. Pat. No. 5,576,568, a floating strip of polysilicon forming the gate electrode of a pass transistor is prolonged and used as the first plate of a capacitor, the second plate whereof is connected to a terminal of a coupling transistor. The polysilicon strip moreover forms the floating gate of an EEPROM cell or a plate of a further coupling capacitor to enable injection or extraction of charges from the polysilicon strip and hence programming and erasing of the cell. Also, these solutions are disadvantageous, since they require a large area and cannot be integrated in a memory array. Programming of the cells moreover requires high voltages, which are hardly from compatible with the devices and circuits integrated in the same chip.